1. Field of the Invention
The present invention relates to a mode switching circuit of a color television set which switches to a suitable mode according to each of the broadcasting systems of PAL-N, NTSC, and PAL-M.
2. Description of the Prior Art
Various types of NTSC, PAL-M and PAL-N systems for television broadcasting systems are well known in the art, and a broadcasting system is different from country to country. Especially, the countries in the Central and South America have different broadcasting systems even though they are adjacent to one another. Each of the television broadcasting systems have its own vertical synchronization frequency and color demodulating process, thereby catching the television broadcasting normally only with a television set suitable for the television broadcasting system. Accordingly, circuits for the mode selection are built in the conventional color television set so that one can select a suitable mode according to a television broadcasting system.
Since the PAL-M and NTSC systems have a vertical synchronization frequency of a 60 Hz and the PAL-N system has that of a 50 Hz, the PAL-N system can be distinguished by detecting the vertical synchronization frequency but the PAL-M system cannot be distinguished from the NTSC system. That is, when a detected vertical synchronization frequency is a 50 Hz, the PAL-N system is determined and the mode suitable for the system can be selected. However, when a 60 Hz is detected as a vertical synchronization frequency, it is recognized to be both of the PAL-M and the NTSC systems.
Accordingly, in case of a vertical synchronization frequency of a 60 Hz, one of the PAL-M mode and the NTSC mode is selected in advance by using a manual selection switch. At this time, when the color television set is in a color state, the selected mode are kept unchanged. When the color television set is not in a color state, a different mode is selected by the manual selection switch. The conventional color television set has a drawback in which one mode for the PAL-N broadcasting system can be automatically selected. However, the others for the PAl-M broadcasting system or the NTSC broadcasting system is to be manually selected.